1. Field of the Invention
The present invention relates to image forming apparatuses using electrophotography, such as copiers, printers, facsimile machines, and digital multi-functional machines employing multi-beam writing.
2. Description of the Background Art
Some electro-photographic image forming apparatuses use a multi-beam writing system to form images. In such image forming apparatuses, banding may appear in the formed images due to reciprocity law failure, a phenomenon in which the law of reciprocity (“exposure amount=light intensity×time”) is not established, resulting in image quality deterioration. Banding occurring as a stripe pattern may cause uneven image density. A similar problem may also occur in image forming apparatuses using a vertical cavity surface emitting laser (VCSEL), which have been recently developed and are now commercially available.
Conventionally, the effect of reciprocity law failure may be reduced by adjusting the light intensity of light emitting elements disposed at the edge portion of the laser generator. For example, JP-2003-182139-A discloses a multi-beam writing system, in which uneven image density may occur at a sub-scanning direction boundary area between adjacent main scanning lines. To set the image density at the sub-scanning direction boundary area to the same image density as that of the image formed in the same scan, the light intensity of light emitting elements at the edge portion in the sub-scanning is set smaller than the light intensity of light emitting elements at the center portion in the sub-scanning direction.
Further, JP-2004-106365-A also discloses a multi-beam writing system, in which uneven image density may occur at a sub-scanning direction boundary area between the adjacent main scanning lines formed on a given image bearing face. To set the image density at the sub-scanning direction boundary area to the same image density of one main scanning line when a simultaneous scanning of multi-beam scan is conducted on a given image bearing face, the overlapped or superimposed length of adjacently formed scan lines in the sub-scanning direction is set shorter than a minimum pixel.
JP-2009-040031-A describes an image forming apparatus using a vertical cavity surface emitting laser array, in which a plurality of light emitting elements are arrayed two-dimensionally. The interval or pitch of light emitting elements arranged at the center portion in the sub-scanning direction is set narrower than the interval between light emitting elements arranged at other portions in the sub-scanning direction.
However, when the light intensity adjustment of light source is conducted at the edge portion in the sub-scanning in such conventional multi-beam system, in a case in which the intervals between light emitting elements arranged at the center portion in the sub-scanning direction is set narrower than the intervals between light emitting elements arranged at other portions in the sub-scanning direction in the light source and interlace scanning conducted for writing images, the light intensity adjustment is not be conducted evenly throughout the image, and therefore banding cannot be eliminated from the image.
When a light source having a plurality of light emitting elements is used for foaming an image using an interlace scanning process with higher density writing, although an image having a desired resolution level may be formed using a plurality of light emitting elements the image may be formed with uneven image density due to the effect of reciprocity law failure.
Light intensity can be adjusted to minimize or eliminate such uneven image density. However, adjustment of the light intensity of the light emitting elements at the edge portion in the sub-scanning direction of the light source alone may not be effective to suppress fluctuation of light intensity, by which uneven image density cannot be reduced or eliminated from the image.